The 24-year-old Spaniard only joined the Portuguese outfit from Sevilla last summer and took great satisfaction out of his late goal against the Liga side
The hosts looked set to suffer a damaging first-leg defeat, with Jon Aurtenetxe having given their Basque opponents the lead early in the second half.
However, Emiliano Insua headed Sporting level on 76 minutes before Capel fired home to give the Portuguese outfit a crucial one-goal advantage ahead of next week's trip to San Mames.
"The goal was a special moment," admitted Capel, who arrived at the Jose Alvalade from Sevilla last summer. "I am very pleased.
"We did our work and it was a magnificent night. Sporting played at a higher level and I am very happy with this win."
However, Capel is well aware that Sporting face an onerous task in attempting to preserve their lead in Basque country.
"It will be a very complicated game, very difficult," the former Spain international conceded. "We must continue to do what we have done; to continue to play at this level."
Sporting paid a reported €3.5 million for 24-year-old Capel, who has been capped twice by his country.